The present invention relates to a brake system of the type including a master cylinder connected to wheel brakes and a booster which includes a movable booster wall and, more particularly, to such a system including a restoring arrangement coupled to a pedal.
A brake system of this general type has been described, for example, in German patent document DE-OS 33 17 629. The system substantially comprises a master cylinder and a booster boosting the pedal force acting upon the master cylinder. The wheel brakes, respectively through electromagnetically operated valves, are in communication with the master cylinder. Each respectively connected valve, in its initial position, is opened such that a pressure fluid communication is established between the master cylinder and the wheel brake cylinders. The rotating pattern of the wheels is permanently monitored, thereby immediately detecting once a wheel tends to lock during deceleration. In the event of a locking tendency, the brake pressure in the master brake cylinder is caused to drop, at the same time the valves of the wheels not tending to lock are switched to the blocking position so that it is only the wheel brake cylinder of the wheel tending to lock to which a reduced pressure is applied. The pressure decrease in the master brake cylinder is effected by re-switching the booster. Reswitching means that the chamber of the brake force booster previously in communication with a high pressure source for enhancing the pedal force, will be connected to a low pressure source whereas the chamber previously connected to the low pressure source, therefore will be in communication with the high pressure source. This will result in a relief of the master brake cylinder and, hence, in a pressure drop in the brake circuits as the pedal forces are compensated rather than enhanced.
As described in GB-PS 15 11 254, the problem is that the driving end and, hence, the restoring forces are determined by the effective surface of the booster and by the pressure difference between high pressure source and low pressure source which typically are of a configuration such that in case of a maximum output, that is at the highest possible pressure difference on the booster wall, a pressure is developed in the master brake cylinder that results in wheel locking. Increasing the pressure in the master brake cylinder, in that case, would not result in a further increase in the deceleration of the vehicle thus support of the pedal force beyond the point of maximum boosting is not required.
However, this type of relief of the master brake cylinder may require compensation of those pedal forces that exceed the boosting forces attained at the point of maximum boosting. The restoring forces of the booster are thus required to substantially exceed the supporting forces. While the supporting forces can be adjusted to the locking pressure, the restoring forces must be adapted to the pedal forces the driver is able to apply. Once the pedal forces exceed the restoring forces, the pressure is not adequately decreased in the master brake cylinder thereby precluding a brake slip control. This problem, according to the noted British Patent, is solved in that a second diaphragm plate is provided on the control sleeve of the brake valve so that during brake slip control, for restoring, the effective booster surface is enlarged. The disadvantages is that corresponding space of movement must be provided for the second diaphragm plate resulting in substantial structural space requirements. Moreover, additional valves are required enabling a corresponding amount of pressure fluid to flow into the chambers confined by the second diaphragm plate.
An object of the invention is, therefore, to provide a brake force booster providing adequate restoring forces while requiring simple valve measures and, at the same time, minimal requirements in terms of structural dimensions.